Add currentDtae indicator and add center-date demo, Fix #84

Hello,

I've added a currentDtae variable in scope than we can show a vertical line for today easily, I also added a sample of center date in demo.html. 

here is screenshot of the vertical line for today:
![capture d ecran 2014-08-27 a 18 28 05](https://cloud.githubusercontent.com/assets/114224/4063387/3b62dcba-2e07-11e4-8fc9-16799f3c1002.png)


a demo on plunker [http://embed.plnkr.co/MQ0mflT7N1CUJKjBm0ap/](http://embed.plnkr.co/MQ0mflT7N1CUJKjBm0ap/)

Fix https://github.com/Schweigi/angular-gantt/issues/84